Implement a takeback request command

[cjsaylor]

Allow a player to request a take back via:

@chessbot propose takeback

which would send an ephemeral message to the opposing player to request yes/no. If yes, the game would rollback a move.

[cjsaylor]

As a stop-gap for simplicity, if the request was within 5 minutes of the last move (and the other player hasn't taken a move yet), automatically allow the take-back.